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Real-World Repairs for Real-World NOX Failures 200

uid 27's pic
posted by  Art Benstein on  Fri Dec 10 13:30 CST 2004
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last visit: Fri Oct 17 20:10 CST 2014
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Your speculation is right on if we happened to be talking about Volvo's LH2.0 stuff. The front end is made of two comparators (Bosch chip has same pinout as an LM339 quad comparator) where the lower of the two is set at 0.49V, the common input is biased with 5 microamps sourced from a precision divider set at 0.52V, and the upper comparator set for .58V on the way up. Hysteresis from its output reduces its setting by 10mV on the way down (~500K resistor) to make it clean.

The sensor never really spends any time in the .49-.58 range even when "lazy" or slow rise time. But how quickly the sensor can react to the engine's correction makes a big difference in overall mixture. Here's a correlation between a lazy sensor's output and the CO test point on my LH2.0 car.



It shows the typical sensor contamination where the output fails to sharply define lean, so the motor stays lean longer than it should. But you can also see the circuit does well to clean up the signal.
--
Art Benstein near Baltimore






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